Some of the borders aren't really well defined in that area, and it could depend entirely on who you ask.

I was there in February, right in this area. In Botswana, they said one thing, in Namibia, they said another. In Zambia, no one seemed to have much of an opinion, both Zambia and Zimbabwe are more concerned with the Victoria Falls border area a bit further away. Basically, people in all four countries told me something different, but my evidence is just anecdotal.

By the way, to give you some idea of how strong these borders are, when I was crossing from Zimbabwe into Botswana, the border guards were so drunk our guide just stamped our passports for us. The actual border was a muddy rutted road with a bike lock on the fence separating the two countries.